Having entered everything into the iPhone and gone back into advanced settings to manually enter the Account URL field, the iPhone finally connects to the NAS...

Now I can create calendar entries from my iPhone and checking in DSM file browser all the .ics files are appearing in the /calendar/OurCalendar/ folder.

Second problem, after configuring the iPad it too can create calendar .ics files on the NAS, only the entries for each device cannot be viewed by each other, same goes for any .ics files added to the /calendar/OurCalendar/ folder.

End of the day, Synology said that DSM 4 would fix the issue... and it hasn't.